The present invention relate to a device for reading spectral lines contained in an optical spectrum.
It finds applications notably in the field of optical communications.
The invention applies particularly to networks of sensors with optical fibres.
These comprise notably networks of deformation sensors with optical fibres and very often photo-inscribed Bragg gratings constituting the components transducing deformation (or even pressure or temperature).
One of the first architectures of networks to have been published uses an optical source with a spectral width greater than the spectral band containing the spectra of the Bragg gratings and sequentially analyses the wavelengths reflected by the different sensors (demultiplexing in terms of wavelength and then spectral analysis of the different signals).
In this regard, reference should be made to documents (1) to (4) which, like the other documents cited below are set out at the end of the present description.
Such networks of sensors can be used for monitoring structures in the following fields: building, civil engineering, transportation, aeronautics and aerospace.
Four techniques are known for effecting integrated optics demultiplexing: a first technique using an engraved grating, a second technique using Mach-Zehnder interferometers, a third technique using an array of microguides or PHASAR (standing for PHASe-ARray), and a fourth technique using balanced Mach-Zehnder interferometers or 100% couplers with a Bragg grating photo-inscribed identically on the two arms (xe2x80x9cADD-DROP multiplexerxe2x80x9d).
The first technique uses the diffraction of light by a concave grating (with a circular or plane exit field) engraved and blazed to a high degree.
Vertical engraving is possible in the case of silica on silicon guides and can attain a depth of 25 xcexcm.
In this regard, document (6) should be consulted.
The demultiplexing component then consists of an input fibre connected to a planar guide sending the light in the direction of an engraved diffraction grating.
In the case of a grating with a circular exit field, the incident light and the diffracted light, refocused at different angular incidences, are located on the Rowland circle.
In the case of a plane field grating (see document (6)), the stigmatic points dispersed in terms of wavelength are aligned on a straight line orthogonal to the reflected field.
As the grating functions by reflection, it is metallised.
The engraving profile of the grating can consist of a set of ellipses, as taught by document (7).
The diffracted beam is refocused on monomode guides having for example a mode diameter of 9 xcexcm and a spacing of 16 xcexcm, as taught by document (6), or on photodiodes forming an array as taught by document (5).
The network preferably functions at a high degree of diffraction (ranging from 4 in document (6) to 50 in document (5)) with the intention of effecting a high-density demultiplexing (for telecommunications).
The second technique is based on putting several interferometers of the Mach-Zehnder type in series, which are all unbalanced with regard to their optical paths, with a characteristic imbalance value.
In this regard, document (8) should be consulted.
For a demultiplexer with four channels, two interferometers are for example used, whose imbalances are equal respectively to xcex94L1 and xcex94L2=xcex94L1+xcex/4N, and a third interferometer whose imbalance xcex94L3 is equal to 2.xcex94L1(typically around 50 xcexcm to 100 xcexcm) in order to obtain an inter-channel separation of 7.5 nm to 1,550 nm, N being effective index of the mode.
The third technique uses an optical phase-array which consists of a set of parallel monomode dephasing guides connecting two plane input and output guides by means of circular interfaces.
In this regard, document (9) should be consulted.
Input guides and output guides are connected to the other circular interfaces of the plane guides.
The light injected by any one of the input guides lies in the input guide plane and covers all the dephasing guides situated at the interface.
From one dephasing guide to another, there is a constant difference in length so that the light beams emerging from the output guide plane interfere as if they were reflected by an inclined concave diffraction grating.
The offset in the optical path caused by the dephasing guides produces the same effect as an inclination of the wave front with respect to the interface.
The PHASAR, which functions by transmission, must behave like a grating with a concave diffraction of a very high degree (approximately 50 to 100) and with a high multiplexing capacity.
In this regard, document (10) should be consulted.
The greater the number of dephasing guides, the better the spectral resolution.
For example, in document (11), 60 dephasing guides are used.
In order to cancel out the polarisation dependency of this circuit, one possible solution is to insert a half-wave plate in the middle of the optical circuit formed by the dephasing guides.
The fourth technique uses balanced Mach-Zehnder interferometers or 100% couplers with a Bragg grating photo-inscribed in an identical fashion on the two arms. The light is injected at the port 1 and emitted at the port 3 (100% coupling) for all the distinct wavelengths of the Bragg wavelength; the light at the Bragg wavelength is reflected selectively at the port 2. In this regard, document (29) should be consulted, from which all the references in the description of the fourth technique, given in the present section, are derived.
Three kinds of material are used for producing the components used in the above four techniques:
glass, silica on silicon and semiconductors of the InP type.
In particular, engraved gratings and PHASARs have been produced in integrated optics on silicon whilst demultiplexers with interferometers have been produced in integrated optics on silicon and on glass.
The components by means of which these four known techniques are implemented are only demultiplexers which serve merely to separate different spectral contributions.
These components do not make it possible to determine the Bragg wavelengths directly with the required precision.
In addition, these techniques require a compromise between cross-talk and spectral space occupied.
Cross-talk, that is to say the light coupling between the outputs, must be minimised since it contributes to falsifying the wavelength measurements.
Typically, a cross-talk of xe2x88x9225 dB to xe2x88x9230 dB is sought and the spectral occupation is derived accordingly.
In the case of a diffraction grating in integrated optics on silicon, the light coupling between the outputs is caused by the diffusion in the guide (because of engraving imperfections) and by the coupling between the output guides when these are two close together.
Between the centres of two adjacent spectral channels, the cross-talk is typically around xe2x88x9220 dB to xe2x88x9235 dB whilst it is no more than xe2x88x9210 dB to xe2x88x9215 dB at the intersection of the transfer functions corresponding to these channels (at half the spectral period).
In this case, a spectral space unoccupied by a transducer is therefore necessary so as to guarantee the minimum of cross-talk necessary.
Typically, this cross-talk is achieved with a spectral occupation of around 0.8 nm on 2 nm of period (see documents (5) and (6)).
The characteristics of cross-talk and occupied space of the PHASAR and engraved grating are equivalent.
Typically, a cross-talk better than xe2x88x9230 dB is achieved in the case of document (11), for a spectral occupation of 0.8 nm and a period of 2 nm, with 60 dephasing guides and a degree of diffraction equal to 60.
In the case of Mach-Zehnder interferometers, the cross-talk depends on the accuracy of adjustment of the separation couplers (3 dB couplers).
By way of example, in document (8) a demultiplexer is described which consists of three interferometers formed from 3.1 dB couplers (instead of 3 dB) and which is characterised by a cross-talk of approximately xe2x88x9220 dB.
In document (4) there is also proposed a demultiplexer which includes a device for collimating the light to be analysed and a series of cascaded bandpass filters associated with photodetectors.
The main drawback of this demultiplexer is being designed to operate in free space.
Because of this, the reproducibility and reliability of the measurements and the robustness and integration of this demultiplexer are insufficient for an application to microsystems.
In addition, the minimum cross-talk which it is possible to obtain with this demultiplexer depends on the reflection of the bandpass filters used (which typically have anti-reflection deposits of xe2x88x9220 dB) and also depends greatly on the polarisation of the light analysed (the filters are oriented at 45xc2x0).
Finally, such a demultiplexer does not lend itself to mass production compatible with the requirements of the market for industrial sensors.
The present invention provides for a device (preferably integrated) for reading spectral lines contained in an optical spectrum, this device comprising demultiplexing and measuring devices (preferably integrated) and having a low response time (large passband in terms of frequency) and optimised manufacturing cost and xe2x80x9cflexibilityxe2x80x9d.
This reading device then makes it possible to design Microsystems for measuring deformations which function in real time, in a wide range of frequencies extending up to ultrasonic frequencies.
The principle of operation of this reading device uses techniques developed for optical telecommunications but, unlike what was done in this latter field, the present invention uses not only demultiplexing of different channels (corresponding for example to different sensors) but also allows the measurement of wavelengths corresponding to these channels.
The present invention resolves the problem of design of a spectral line reading device
which is able to have a high multiplexing capacity to permit the simultaneous observation of a large number of transducers, typically eight or more, with a very low light coupling (cross-talk) between its outputs, typically around xe2x88x9225 dB to xe2x88x9230 dB,
which is able to have a large passband in terms of frequency, around 100 kHz for example,
which is able to be integrated on a planar substrate,
which has great flexibility in manufacture (since it permits the adjustment of the tuning wavelengths).
Precisely, the object of the present invention is a device for reading spectral lines which are contained in an optical spectrum and are liable to fluctuate respectively in given spectral domains, this device being characterised in that it comprises:
a device for demultiplexing these spectral lines in terms of wavelength, this demultiplexing device having an input intended to receive the optical spectrum and outputs intended to supply respectively the demultiplexed spectral lines,
a device for measuring, by filtering, the respective wavelengths of the demultiplexed spectral lines, comprising, for each of these lines, a measuring channel provided with a filter and a reference channel, and
means of photodetection, for each spectral line, of the light intensities respectively transmitted by the corresponding measuring and reference channels, so as to be able to determine the wavelength of this line by calculating the ratio of the intensities thus detected.
The device which is the object of the invention is thus an optical device able to be integrated on a planar substrate making it possible to obtain a set of electrical signals representing the wavelengths of the spectral lines (each of these lines being sufficiently separated from the adjacent spectral lines).
These spectral lines can for example result from the reflections of transducing Bragg gratings photo-inscribed on a sensitive monomode optical fibre, subjected to stresses or to variations in temperature and pressure.
More generally, these lines can result from transducers which generate spectral lines whose positions it is sought to determine.
The originality of the device which is the object of the invention lies notably in the fact that it incorporates (preferably on one and the same substrate) a demultiplexing part and a filtering/measuring part.
The device for demultiplexing in terms of wavelength can be a demultiplexing device with an engraved grating or a network of micro-guides (or PHASAR, already considered above).
However, according to a preferred embodiment of the device which is the object of the invention, the demultiplexing device comprises:
an energy separator, having an input which is intended to receive the optical spectrum, and a plurality of outputs which are capable of supplying respectively fractions of the light energy of the optical spectrum, and
a plurality of wavelength-selective light reflectors which are respectively connected to the outputs, each wavelength-selective light reflector having a wavelength passband which contains the spectral region associated with one of the lines and therefore reflecting only this line, each selective reflector being connected to an optical waveguide intended to propagate the line reflected by this reflector.
It is possible to use an energy separator of a known type, for example of the type sold by Corning. This energy separator can be a set of separating junctions mounted in cascade, that is to say in a tree (their mounting is as a tree structure).
Each separating junction can be multimode but is preferably monomode notably when the optical spectrum which is to be demultiplexed is conveyed by a monomode optical fibre, connected to the input of the device.
These separating junctions can be couplers, for example 3 dB couplers (couplers which are such that each of their two output channels transports half of the incident light energy).
However, this makes it necessary to precisely adjust the coupling length and the interval between the coupled waveguides of such couplers according to the wavelength.
This is why, in the present invention, it is preferable to use separating junctions consisting of Y junctions.
These Y junctions have the advantage of being achromatic and independent of the polarisation.
One advantage of the preferred embodiment of the device which is the object of the invention lies in the excellent rejection in terms of wavelength, which ensures very low cross-talk (spectral profile of reflection in square waves), and in the very great xe2x80x9cflexibilityxe2x80x9d in manufacture.
Compared with the device described in document (4), the device which is the object of the invention has the advantage of using no collimation device and allows integration of all its basic components (filters, separators etc), the light remaining guided at all points in the device.
The latter thus makes it possible to resolve the problems posed above, namely achieving a high multiplexing capacity with very low light coupling between outputs (very low cross-talk) with a large passband in terms of frequency, whilst ensuring a cost, integration, robustness and flexibility in manufacture (wavelength adjustment) which are compatible with the requirements of the instrumentation and sensor market.
In the preferred embodiment of the device which is the object of the invention, the selective reflectors can comprise Bragg gratings. These Bragg gratings can be photo-inscribed or photo-engraved.
In addition, these Bragg networks can be variable-period gratings (chirped gratings).
It is also possible to use gratings with a fixed period and maximum reflectivity (xe2x80x9csaturatedxe2x80x9d), which is achieved for example for photo-inscription under very high flux (so as to broaden their spectral response).
The filters respectively associated with the measuring channels can be Chirped Bragg gratings or filters with dielectric multilayers different from each other or filters with dielectric multilayers identical to each other or Mach-Zehnder interferometers.
The demultiplexing device and device for measuring by filtering can respectively be integrated on two substrates which are connected by means of optical fibres intended to transmit the demultiplexed lines from the demultiplexing device to the device for measuring by filtering.
These two substrates can be made of glass or silicon, or III-V semiconductor (such as for example AsGa or InP).
However, the demultiplexing device and the device for measuring by filtering are preferably integrated on the same substrate, which can be made of glass or silicon, or from III-V semiconductor.
According to a preferred embodiment of the invention, which is highly integrated, the demultiplexing device, the device for measuring by filtering and the means for the photodetection of spectral lines are integrated in the same substrate made of silicon or III-V semiconductor.
The device which is the object of the invention can also comprise an optical fibre which is optically coupled to the input of the demultiplexing device and which is intended to bring the optical spectrum to this entry.